Patterned pavement marking

ABSTRACT

A retroreflector sheet comprising a base sheet and integral protrusions having a top and a side surface. Selected side surfaces are covered with a bead bond layer including partially embedded retroreflector beads. A process for preparing the retroreflector sheet is also disclosed.

BACKGROUND OF THE INVENTION

The present invention is a new method for preparing pavement-markingsheet material.

Pavement-marking sheet material which is economical and performs well inboth daylight and night conditions is a continuing goal of thepavement-marking industry. Night performance is primarily provided byretroreflection, which may be defined as a phenomenon in which a largeportion of luminous radiation is returned in the direction from which itoriginates. Spherical retroreflectors, typically tiny glass beads ormicrospheres, are well known in the industry.

Flat single layer, polymeric sheet material, as well as flat laminatedmaterials, are known to hold the beads in position. The sheet materialis applied to a highway surface and serves to both cushion and hold theretroreflective beads. The efficiency of flat pavement-markingmaterials, however, is limited for two reasons. First, the exposedsurfaces of the beads are directed upward, whereas the optimalorientation is toward vehicle headlights which typically illuminate theretroreflective beads from angles slightly above the road surface.Second, in an upwardly directed fashion, the exposed surfaces of thebeads are exposed to maximum abrasive wear by vehicle tires, thusallowing rapid destructive abrasion of the exposed surface.

An alternative approach is to provide a raised pattern on the pavementmarking strip. A raised pattern includes three advantages. First, araised pattern encourages runoff of rain water. Second, a raised patternprovides nonhorizontal surfaces to support retroreflective beads. Thenonhorizontal surface isolates the optical surfaces of the beads fromabrasive wear by traffic and provides a more effective orientation ofthe retroreflective beads. Third, a raised pattern allows the use ofhighly efficient specular retroreflecting beads.

U.S. Pat. No. 4,069,281 discloses a prefabricated roadway marking stripmaterial including a base layer and a traffic-regulating, sign-formingand traffic wear-resisting upper layer having a generally smoothsurface. Spaced protuberances bulge from the surface and retroreflectiveelements are concentrated on the top portion of the protuberances. Theprotuberances are thick portions of the generally smooth upper layer.

Another road marking material is described in U.S. Pat. No. 3.935,365.The material is formed by embossing spaced transverse protrusion onextruded stock. Beads are placed on each transverse protrusion toprovide reflectorized flanks.

An alternative product is disclosed in U.S. Pat. No. 4,681,401(Wyckoff). The product includes flattened, somewhat saw-toothed shapedwedges embodying retroreflective material and of preferablysubstantially trapezoidal shape.

Another method of forming a raised pattern on a pavement-marking sheetmaterial is disclosed in U.S. Pat. No. 4,388,359. The preparation of thematerial includes depositing a monolayer of microspheres on a base sheetand embossing the microsphere-covered base sheet so as to deform thebase sheet and form protuberant areas separated by depressed areas. Theembossing step partially embeds the glass microspheres into the basesheet in the protuberant areas and can fully embed the microspheres intothe base sheet into the depressed areas.

SUMMARY OF THE INVENTION

The subject invention is a process for producing a retroreflector sheet.The process steps include providing a resilient polymeric base sheethaving a plurality of protrusions, applying a discontinuous layer ofliquid bead bond to selected side surfaces of the protrusions, partiallyembedding retroreflecting beads in the liquid bead bond, and solidifyingthe bead bond. The invention also is a retroreflector sheet having aresilient base with integral protrusions. Selected side surface of theprotrusions are covered with a layer of bead bond in whichretroreflector beads are embedded.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a base sheet with protuberances.

FIG. 2 is a top view of the base sheet and protuberances.

FIG. 3 is a schematic view of the process.

FIGS. 4A, 4B, 4C, 4D and 4E are cross-sectional views of embodiments ofthe subject invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The subject invention is an efficient process for producing a highlyeffective retroreflector sheet useful for both daytime and nighttimemarking purposes. The resulting retroreflector sheet is particularlywell suited for use as a pavement marking strip.

The process involves four main steps. First, a resilient base sheet withresilient protrusions is provided. Second, a liquid bead bond layer isselectively applied to desired surfaces of the protrusions. Theremaining surfaces of the protrusions and base sheet remainsubstantially free of bead bond. Third, retroreflective beads or,alternatively, other useful particles, such as skin-preventingparticles, are embedded in the liquid bead bond layer. Fourth, theliquid bead bond layer is solidified, thereby stabilizing the locationof the embedded particles. Variations in the components employed andvariations in process step parameters can produce an array of usefulproducts. Additionally, certain process steps may be repeated in thecourse of the process using alternative parameters and alternativecomponents to further broaden the array of useful products.

The first major component required to carry out the process is aresilient polymeric base sheet having a plurality of protrusionsprojecting from a surface of the sheet. The tops of the protrusionsessentially define a plane substantially parallel to the surface of thesheet. The protrusions need not necessarily be regularly shaped, sized,or spaced apart for the process to successfully apply liquid bead bondto selected surface portions. However, the process of this invention ismore easily understood and explained with reference to the preferredembodiment which includes regularly shaped, sized and spaced integralprotrusions projecting from a base sheet.

A suitable base sheet may preferably be formed using known methods andmaterials, such as described in U.S. Pat. No. 4,388,359, which isincorporated herein by reference. The embossed base sheet described inthe patent comprises elastomer precursors, not yet vulcanized or cured,which therefore permit viscoelastic deformation. Exemplary materials areacrylonitrite-butadiene polymers, millable urethane polymers andneoprenes. Extender resins may be included. Particulate fine-diameterfillers such as silica, asbestos, etc. may be included, however,environmental health considerations may advice against asbestosincorporation. Pigments, such as titanium dioxide are preferred in thebase sheet to provide a white diffuse surface to uncoated portions ofthe base and protrusions. Another useful pigment is lead chromate whichimparts a yellow color. The base sheet, however, departs from thedisclosure of U.S. Pat. No. 4,388,359 in that a mono-layer oftransparent microspheres is not deposited prior to embossing. In onealternative embodiment, however, skid prevention particles are partiallyembedded in the base sheets during embossing.

The preferred resilient polymeric base sheet, preferably a web, isgenerally shown as 100 in FIG. 1. The sheet 100 includes a base 102 anda plurality of protrusions 104. The protrusions 104 are an integral partof the base sheet 102 and have a top surface 106 and a side surface 108.The protrusions 104 typically have a height of approximately 1.1 mm(1.0, 1.8 and 2.5 mm alternative embodiments have also been tested). Thebase 102 has a front surface 103 from which the protrusions extend and aback surface 105 and typically has a thickness of approximately 0.64 mm.The side surface 108 meet the top surface 106 at a rounded top portion110. The side surface 108 meet the front surface 103 at a lower portion112. The side surfaces 108 may form an angle θ of approximately 70°-72°at the intersection of front surface 103, with the lower portion 112 ofthe side surface 108.

The protrusions 104 are disposed upon the base 102 in rows 114 andcolumns 116, each oriented at about 45° to an edge 118 of the base sheet102 as shown in FIG. 2. If the base sheet 100 is a web, then an upwebdirection 119A and a downweb direction 119B are also present. By "upweb"is meant the direction of web portions to which bead bond has not yetbeen applied. By "downweb" is meant the direction of web portions towhich bead bond has previously been applied. The protrusions 104 have agenerally square outline such that the side surface 108 of each of theprotrusions 104 is divided into four roughly equal parts, each having atop portion 110. Two of the top portions 110A face the upweb direction119A and two of the top portions 110B face the downweb direction 119B.The length of the portions 110 of the protrusions 104 is typically about6.4 mm. The rows 114 and columns 116 are spaced apart a distanceapproximately 3.2 mm, however, alternative spacings up to about 6.4 mmhave also been successfully tested.

In the second step of the process, a layer of liquid bead bond materialis applied to selected portions of the protrusions 104. In a mostelementary form of the second step, a film of liquid bead bond iscontacted with the top surface 106 of the protrusions 104. The secondstep may be carried out by employing a film of liquid bead bondsupported by a release liner or sheet. Portions of the film arelaminated to portions of the protrusions. Subsequently the release lineris stripped off or removed, thereby leaving the protrusion surfacesselectively printed with liquid bead bond. If the release liner ispressed hard against the top surface 106 of the protrusions 104, theliquid bead bond is displaced from the top surface 106 to the sidesurface 108.

A more sophisticated continuous process involves a film of bead bondsupported on a roller.

The continuous process for applying transparent bead lens elements tothe base retroreflecting sheet is schematically presented in FIG. 3. Theweb 100 of base sheet material is oriented with the protrusions 104projecting downward from the surface 103 and back surface 105 orientedupward. The protrusions 104 contact a film 220 of liquid bead bond. Thefilm 220 is provided by a print roller 222 which is partially immersedin a reservoir or pool of liquid bead bond 224. A backing roller 226contacts the back surface 105 of the web 100. As the print roller 222rotates through the reservoir of liquid bead bond 224 the film 220 isformed on the print roller. As the rotation continues, the film 220contact the protrusions 104. The web 100 is also advanced by therotation of the backing roller 226. As the protrusions 104 contact thefilm 220, a discontinuous layer of bead bond 228 is applied to orprinted on the protrusions 104. Non-adhering portions 221 of the film220 returns to the reservoir 224 on the print roller 222.

Suitable bead bond material may be either a thermoplastic orthermosetting polymeric binder. One such binder is a vinyl-basedthermoplastic resin including a white pigment, as described in U.S. Pat.No. 4,117,192 incorporated herein by reference. Other suitable bead bondmaterials include two-part polyurethane formed by reactingpolycaprolactone diols and triols with derivatives of hexamethylenediisocyanate; epoxy based resins as described in U.S. Pat. Nos.4,248,932, 3,436359, and 3,580,887; and blocked polyurethanecompositions as described in U.S. Pat. No. 4,530,859. Also suitable as abead bond material are polyurethane compositions comprised of a moistureactivated curing agent and a polyisocyanate prepolymer. The moistureactivated curing agent is preferably an oxazolidene ring. Suchcompositions are described in U.S. Pat. No. 4,381,388.

The preferred polyurethane bead bond is formed by first reacting twoequivalents of methylene bis (4-cyclohexyl isocyanate) (H₁₂ MDI) withone equivalent of a polycaprolactone triol of molecular weight about 540and hydroxyl number about 310 (i.e., a 2-oxypanone polymer with2-ethyl-2-(hydroxymethyl)-1,3 propanediol) using dibutyltindilaurate asa catalyst. The reaction is carried out in 2-ethoxyethyl acetate andcyclohexanone. To 25 parts of prepolymer is also added 20 parts of a60/40 pigment dispersion of either titanium dioxide or lead chromate ina diglycidyl ether of bisphenol A epoxy resin (a suitable source isStan-Tone® 10 EPXO3 or 30 EPXO3 made by Harwick Chemical Corp. of Akron,Ohio). Zinc 2-ethylhexanoate catalyst is added to the bead bond mixtureshortly before printing. Inclusion of up to about 10% 2,4 pentanedionein the preferred bead bond extends the pot life of the bead bond fromabout b 1.5 hours to about 15 hours without affecting bead retention.

Useful ranges of pigment dispersion which may be included are 10-30parts per 25 parts of urethane prepolymer. Hydrogenated epoxies may alsobe employed. Other useful pigments include nacreous pigments, such aslead sulfate; specular reflectors, such as metallic (for examplealuminum) powder or flakes, as well as yellow iron derived pigments.Other pigments typically used for coloring pavement markings may also beused.

Generally, suitable bead bond materials, such as described above, arecharacterized by excellent adhesion to beads or particles which aresubsequently embedded in the bead bond layer. Additionally, the beadbond layer strongly adheres to the base sheet material, is highlycohesive and resistant to environmental weathering.

After the bead bond is applied, the web is inverted such that theprotrusions 104 including the selectively printed layer of bead bond 228face upward. Retroreflecting beads 120 are applied to the web 100 andbecome partially embedded in the still liquid bead bond 228. The beads120 may be applied by a flood coating process which results in a densepacking of retroreflecting beads 120 in the surface layer 228 of liquidbead bond. Alternatively, the beads may be sprinkled or cascaded uponthe web 100 such that a dense packing of retroreflecting beads 120 inthe liquid bead bond 228 is avoided. The sprinkling process isespecially advantageous for further decreasing bead usage and furtherdecreasing dirt retention between beads.

Particles such as retroreflective beads suitable for use in the processinclude glass beads formed of glass materials having indices ofrefraction (n) from about 1.5 to about 1.9. As is well known in the art,glass beads of material having an index of refraction of about 1.5 areless costly and more scratch and chip resistant than glass beads ofmaterial having an index of refraction of from about 1.75 to about 1.9.However, the cheaper, more durable glass beads are less effectiveretroreflectors. In one embodiment, the glass beads may include a silveror other specular reflective metallic or dielectric coating. Thenon-embedded portion of the silver coat is subsequently removed toprovide a highly effective retroreflector. In another embodiment, beadshaving a hemispheric coating of a specular reflective metal, such assilver, are applied to the liquid bead bond layer. Because the beads arerandomly oriented when applied, a fraction of the beads because embeddedin an orientation which is effective for retroreflection. Generally, theeffectively oriented beads have the uncoated surface exposed and thesilver coated surface embedded.

Preferred retroreflector beads are disclosed in U.S. Pat. Nos. 4,564,556and 4,758,469. This will reflect the issuance of Ser. No. 35,989, whichare incorporated herein by reference. They are described generally assolid, transparent, non-vitreous, ceramic spheroids comprising at leastone crystalline phase comprised of at least one metal oxide. They mayalso have an amorphous phase such as silica. The term non-vitreous meansthat they have not ben derived from a melt or mixture of raw materialsbrought to liquid state at high temperature, like glass. These spheroidsare very resistant to scratching and chipping, being quite hard (e.g.,above 700 Knoop) and they can be made with a relatively high index ofrefraction (ranging between 1.4 and 2.6). Examples of the compositionsof these beads are zirconia-alumina-silica and zirconia-silica.

The retroreflector beads preferably have a diameter compatible with thesize, shape, spacing and geometry of the protrusions present upon thebase sheet. For the earlier described base sheet 100 beads of from50-350 micrometers diameter may be suitable employed. Other factorsaffecting bead size are the number of rows of beads desired to beavailable to vehicle headlights. At an angle of about 2°-3° from thebase sheet 100 only about 380 micrometers of side surface is visible.Thus, only about 1 row of 300 micrometer beads is visible, or about 2rows of 225 micrometer beads. Preferably, 225 micrometer beads areemployed in order to reduce the criticality of bead retention.

The approximate weight of typical ceramic beads with a density ofapproximately 4.0 grams per cubic centimeter, corresponding to floodcoatings (i.e., a monolayer of beads over all surfaces except the backsurface 105) of the entire surface of the protrusion 104 and frontsurface 103 of the base sheet 102 is given in Table I. Levels of beadapplication for selectively applied beads range from just greater than0% to about 100% of flood coat. Preferred levels, however, are from15-50% of flood coat, with about 30% being most preferred. Anotherconsideration is the relationship of the bead bond layer to bead size.Unlike flat pavement marking constructions, beads will retroreflect onthe side surfaces of the protrusions when deeply embedded, as long as aportion of the bead surface is exposed. Preferably, beads should beembedded up to approximately 50-70% of their diameter in the liquid beadbond layer for an acceptable compromise between bead retention in thefiled and ability to retroreflect light. Retention of glass beads mayalso be improved by silane treatment.

Additionally, the process is applicable to selective embedding of skidprevention particles such as sand, Al₂ O₃, etc.

                  TABLE I                                                         ______________________________________                                        Bead Size      100% Flood Coat                                                (Micrometers)  (kg/m.sup.2)                                                   ______________________________________                                         50-100        0.14                                                           100-150        0.21                                                           100-200        0.25                                                           150-200        0.31                                                           200-250        0.40                                                           ______________________________________                                    

In the fourth step of the process, the liquid bead bond is solidified,thereby locking the retroreflecting beads in the partially embeddedposition within the bead bond layer. The particular solidification stepemployed depends on the nature of the bead bond. For polyurethane typebead bonds, solidification may be accomplished by elevation of thetemperature to encourage a polymerization reaction. Alternative means ofsolidifying the bead bond such as cooling of a thermoplastic bead bondare also envisioned. For the preferred polyurethane bead bond, atemperature of about 177° C. for approximately 10 minutes results inrapid solidification of the bead bond and excellent bead retentionproperties in the field. An oven 300 is useful for providing thetemperatures to cause solidification. For the continuous process, theoven may be a tunnel arrangement.

Retroreflecting beads contacting the web 100 but not encountering theliquid bead bond may subsequently be removed by either an inversion stepwith vibration or a vacuum step thereby allowing non-embedded beads tobe saved and reused.

A number of critical factors in the process of forming theseretroreflective sheets determine both the effectiveness of the processand the nature of the product which will be produced. Particularlyimportant among these parameters are pressure applied at the nip 302 andspeed of the web 100 relative to the film 220. By "nip" 302 is meant theregion between the rollers 222 and 226 through which the web 100 passes.

Nip pressure controls the displacement of liquid bead bond from the topsurface 106 to the side surface 108. By "nip pressure" is meant thepressure between the rollers 222 and 226 when the web 100 and bead bondare included. Increasing the nip pressure causes the protrusions 104 toproject through the bead bond film 220 and essentially contact or nearlycontact the print roller 222, as shown in FIG. 3. The contact betweenthe protrusions 104 and the print roller 222, results in a displacementof the liquid bead bond from the top surface 106 of the protrusion 104.The displaced bead liquid contacts the side surfaces 108 of theprotrusions 104 and remains as a layer on the side surfaces 108 of theprotrusions 104 when the web 100 is separated from the print wheel 222.

Several factors affect transfer of bead bond onto the side surfaces,such as adjustment of nip pressure and the hardness of the surfaces ofthe print roller 222 and the backing roller 226. The backing roller 226should preferably be hard material such as steel, although hard rubberrollers having a surface harder than about 75 Shore A units may beemployed. Insufficiently hard rollers absorb a potion of the nippressure. Insufficient nip pressure results in incomplete displacementof bead bond from the top surface 106. Excessively soft print rollers222 and low nip pressure results in application of bead bond over theentire surface of the protrusions 104. Excessively soft print rollersand moderate nip pressure result in application of bead bond over boththe front surface 103 and the entire surface of the protrusions 104. Ineffect, selectivity of application of bead bond is lost by employingexcessively soft print rollers 222.

Other factors are the viscosity of the liquid bead bond and thethickness of the bead bond film 220 upon the print roller 222.Generally, any liquid bead bond with a viscosity suitable for coatingmay be applied by this process. In the particular case of the webpattern earlier described, a polyurethane based bead bond 220 preferablyhas a viscosity at 24° C. of between about 1800 and about 10,000 cps asmeasured on a Brookfield Viscometer. Most preferably, the viscosity at24° C. should be from about 3000 to about 7000 cps.

The thickness of the film 220 on the print wheel 222 should be fromabout 200 to about 500 micrometers thick and preferably about 300micrometers thick. However, the thickness of the film 220 should besufficient to provide a wet liquid bead bond layer on the selectedsurfaces of the protrusions of suitable thickness and viscosity suchthat the subsequently applied retroreflecting beads may be embedded to adepth of about 40-60% of the diameter of the bead. Thus, the thicknessof the film 220 may be partially determined by the bead thickness. Adoctor bar 304 may be employed to regulate the film thickness. Apreferred doctor bar 304 is made of spring steel and is spaced apartfrom the print roller 222. Alternatively, a third roller may be used toregulate film thickness.

The speed of the web 100 relative to the speed of the film 220 controlsthe application of the liquid bead bond on upweb or downweb portions 110of FIG. 2 of the protrusions 104. In a first situation, the film 220travels faster than the protrusion 104, and the nip pressure is suchthat the top surface 106 contacts the print roller 220. Additional beadbond accumulates on the upweb portions 110A of the protrusion 104 andthe space between the tops 106 and the print roller 222. As the webadvances into the nip 302, the additional bead bond is displaced to thesides 108 including upweb portions 110A. The downweb facing portions110B of the sides 108 and the tops 106 remain substantially free from alayer of liquid bead bond.

Alternatively, when the protrusions 104 move faster than the film 220and contacts the print roller 222, the protrusions 104 tend to sweep orwipe the liquid bead bond onto the sides 108 including downweb portions110B of the protrusion 104 while the upweb portion 110A of the side 108and the top 106 remains substantially free from a layer of bead bond.

A product 200 of the process of this invention, is illustrated in FIG.4A, with retroreflecting beads 120 partially embedded in a bead bondlayer 122 on the side 108 including the op portions 110.

An alternative embodiment 202, as shown in FIG. 4B, includes skidpreventing particles 124 partially embedded in the top 106 of theprotrusions 104 in addition to the retroreflecting beads 120 partiallyembedded in the bead bond layer 122 on the side surface 108 includingthe top portions 110. Production of the embodiment 202 may beaccomplished by employing a web 100 with skid prevention particlesembedded into the surface.

In a third alternative embodiment 204, as shown in FIG. 4C, theretroreflecting beads 120 are partially embedded in a bead bond layer130 on a portion of the side 108 including the top portion 110A. Theembodiment 204 may be produced by moving the web 100 approximately 10%slower than the bead bond film 220 through the nip 302. The differencein speed between the relatively slower web 100 and the relatively fasterbead bond film 220 results in selective application of the bead bondlayer 130 on the upweb portions of the protrusions. The embodiment 204is particularly advantageous for use as a specialized pavement markersince retroreflection is possible from a a first direction but not froma second direction.

In another embodiment 206, as shown in FIG. 4D, retroreflecting beads120 are partially embedded in a first bead bond layer 132 facing a firstdirection and a second bead bond layer 134 facing a second direction.The embodiment 206 is made by a first pass through the nip 302 with afirst speed differential and a second pass through the nip 302 with asecond nearly opposite differential speed between the web 100 and thefilm 220. Because the pigments of the bead bonds 132 and 134 contributeto the color of the retroreflected light, a first retroreflected lightcolor may be reflected in a first direction and a second retroreflectivecolor of light may be retroreflected in a second direction. Thedifferent colors require different colored pigments in the bead bond.Most preferred are TiO₂ pigment to provide a white retroreflector andlead chromate pigment to provide a distinct yellow retroreflector forthe second direction. Other color/pigment combinations may be used toprovide alternative signal information to drivers.

In another embodiment 208, as shown in FIG. 4E, a third layer of beadbond 136 is subsequently applied to the top of the protrusions of theembodiment 206 of FIG. 4D. The third layer of bead bond 136 is appliedto the top of the protrusions by eliminating the nip pressure oroperating the rollers at a fixed gap. The top layer is suitable forembedding particles 126, such as skid prevention particles orretroreflecting beads.

EXAMPLE 1

A 10.2 cm by 30.5 cm portion of preferred base sheet was provided. Thebase sheet was passed through the nip of a gravure type coater toselectively apply bead bond. The bead bond was a vinyl solutionincluding a white pigment (TiO₂). A doctor blade was used to regulatethe thickness of the film. Immediately after application of the beadbond, the base sheet was flood coated with 165 micrometer diameter, 1.75index of refraction beads. The white vinyl bead bond was solidified byheating in an oven at approximately 121° C. for 10 minutes. Excess beadswere removed by brushing. The resulting product had beads and bead bondselectively applied to pairs of protrusion walls facing a firstdirection and was relatively free of beads and bead bond in a seconddirection. The product demonstrated good qualitative retroreflectionfrom the first direction and little or none from the second direction.

EXAMPLES 2-16

Examples 2 and 3-16 are presented in Table II. For each example, thetable presents base sheet pattern dimensions, type of bead bond and filmthickness and the type and density of retroreflective beads. Examples2-10, 15 and 16 employed beads 165 micrometer in diameter. Examples11-14 employed beads 525 micrometer in diameter. Example 2 is acomparative example of pavement marking material disclosed in U.S. Pat.No. 4,388,359 in which beads are embedded into the retroreflectivesheet. Application of bead bond is examples 3-14 was carried out bycoating a release sheet with a film of the liquid bead bond solution.The coated release sheet was immediately inverted and contacted with thetops of the protrusions. A hand roller was used to squeeze the releasesheet against the protrusions. The release sheet was stripped off andthe base sheet flood coated with the retroreflective beads. The specificluminance was measured subsequent to solidification of the bead bond.Examples 15 and 16 were produced using the continuous process depictedin FIG. 3.

As indicated by the results in Table II, all the examples exceeded thespecific luminance of comparative example 2, while employing far fewerbeads. The examples demonstrate that the invention is useful forselective application of many types of retroreflectors includingspecular reflective beads (examples 13 and 14) and ceramic beads(examples 8-10, 15, and 16). The examples further demonstrate that theinvention is useful with both vinyl and urethane bead bonds and is notlimited to a single base sheet pattern. The most important point of theexamples in Table II is that the beads have been selectively applied tosurfaces which are highly effective in providing retroreflection whenilluminated at high incidence angles.

                  TABLE II                                                        ______________________________________                                        Base Sheet Bead Bond  Beads    Sp. Luminance.sup.6                                 Pattern.sup.1 /                                                                         Type.sup.3 /                                                                             n/     86.0/  86.5/                                 Ex.  Depth.sup.2                                                                             Thick.sup.4                                                                              %.sup.5                                                                              0.2°.sup.7                                                                    1.0°.sup.8                     ______________________________________                                        2    6.4*3.2   N          1.75   1090    680                                       1100                 100%                                                3    6.4*3.2   V          1.75   4220   2500                                       1000      300        50%                                                 4    6.4*3.2   V          1.75   3560   2270                                       1800      300        35%                                                 5    6.4*3.2   V          1.75   2350   1530                                       2500      300        13%                                                 6    6.4*2.4   V          1.75   4440   2650                                       1000      300        50%                                                 7    6.4*6.4   V          1.75   3260   2120                                       1000      300        25%                                                 8    6.4*3.2   U          1.76.sup.9                                                                           1680    980                                       1000      250        50%                                                 9    6.4*3.2   U          1.76.sup.9                                                                           1780   1060                                       1800      250        35%                                                 10   6.4*3.2   U          1.76.sup.9                                                                           1450    850                                       2500      250        13%                                                 11   6.4*3.2   V          1.9    12,700 4780                                       1000      500        25%                                                 12   6.4*3.2   V          1.9    14,300 5430                                       1800      500        15%                                                 13   6.4*3.2   V          1.92.sup.10                                                                          22,500 8070                                       1000      400        25%                                                 14   6.4*3.2   V          1.92.sup.10                                                                          40,200 13,900                                     1800      500        15%                                                 15   6.4*3.2   U          1.76.sup.9                                                                           4130   2420                                       1100      300        35%                                                 16   6.4*3.2   U          1.92.sup.9                                                                           7190   2850                                       1100      300        35%                                                 ______________________________________                                         Table II Notes:                                                               .sup.1 Pattern dimensions as wall length of square protrusions in mm *        space between protrusions in mm.                                              .sup.2 Depth of embossed pattern, or height of protrusions in micrometers     .sup.3 Composition of bead bond:                                              "V" indicates white vinyl as in U.S. Pat. No. 4,117,192                       "U" indicates polyurethane preferred composition described hereinabove.       "N" indicates no bead bond present in comparative example using material      of U.S. Pat. No. 4,388,359.                                                   .sup.4 Film thickness in micrometers.                                         .sup.5 Percent of flood coat on surface as indicated by bead weight gain      of example divided by bead weight gain of totally coated surface.             .sup.6 Specific luminance, defined in Federal Test Method Standard 370,       section 3.1.2, is a photometric quantity used to specify the performance      of retroreflective materials.                                                 .sup.7 Specific luminance of example when illuminated at 86.0° to      perpendicular to top surface and observed at a divergence angle of            0.2° to illumination angle. Expressed in mcd/m.sup.2 /lx.              .sup.8 Specific luminance of example when illuminated at 86.5° to      perpendicular to upper surface and observed at a divergence angle of          1.0° to illumination angle. Expressed in mcd/m.sup.2 /lx.              .sup.9 Ceramic type beads as described in U.S. Pat. No. 4,758,469             .sup.10 Beads having a refractive index of 1.9, coated with silver.           Exposed surfaces etched to remove silver subsequent to solidification of      bead bond.                                                               

EXAMPLE 17

A 10.2 cm wide preferred web as previously described was provided. Thepreferred polyurethane bead bond, at a viscosity in the range ofapproximately 4000-6000 cps, was applied by a continuous process. Thefilm thickness on the print roller was approximately 300 micrometers. Auniform distribution of bead bond layer between the upweb and downwebportions of the protrusions was accomplished by having the web speedexceeding the film speed by approximately 3%. To apply the bead bondlayer substantially on the sides including downweb portion 110B, the webspeed exceeded the film speed b approximately 10%. In order to apply thebead bond layer substantially to the sides including upweb portion 110A,the web speed was slowed to approximately 90% of the film speed. Theproducts produced resembled FIGS. 4A and 4C.

The subject invention provides several advantages over the prior art.First, because the retroreflecting beads are applied only to the area inwhich they are most effective, bead usage per unit area of material issubstantially decreased. Decreased bead usage not only allows thepossibility of saving money on standard beads such as 1.5 refractiveindex beads, 1.75 refractive index beads or 1.9 refractive index beads,but also allows more efficient beads to be employed in retroreflectivesheets. For example, ceramic based retroreflective beads as described inU.S. Pat. No. 4,758,469, may be employed.

An additional advantage of selective application of beads to selectedsurfaces of the base sheet is an ability to avoid extensive "gray cast"areas on the retroreflecting material. Such "gray cast" areas are causedby partially exposed beads on the base sheet. Because bead bond isapplied only as a discontinuous layer to selected surfaces, surfacesfree of bead bond demonstrate a daytime color unaffected by beads; thatis, free of a "gray cast." Minimizing "gray cast" areas has beenreported in U.S. Pat. No. 4,388,359 to maintain the desired white orother daytime color of the sheet material.

The product is more flexible than the 100% bead bond coated base sheet.The conformance and flexibility are substantially the same as anuncoated web 100. Improved conformance allows for easier application toroads.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A retroreflective sheet comprising:a resilientpolymeric base sheet having a front surface; a plurality of integralprotrusions projecting from the front surface, there being a pluralityof such protrusions across the width and down the length of the sheet,each of the protrusions having a top surface and at least one sidesurface connecting the top surface to the front surface of the basesheet; a first discontinuous layer of bead bond covering a selected setof side surfaces of each of the protrusions; and a first plurality ofretroreflecting beads partially embedded in the layer of bead bond andpartially protruding from the layer of bead bond, said retroreflectivesheet being characterized in that said selected set of side surfaces iscovered by a combination of bead bond and retroreflecting beads absentfrom the remaining front surface between the protrusions.
 2. Theretroreflective sheet of claim 1 wherein the protruberances aresubstantially identical and are disposed in a substantially regularrepeating pattern upon the base sheet.
 3. The retroreflective sheet ofclaim 1 wherein the retroreflecting beads are non-vitreous, ceramicspheroids comprising at least one crystalline phase comprised of atleast one metal oxide.
 4. The retroreflective sheet of claim 3 whereinthe retroreflective beads comprise at least one polycrystalline phaseand at least one amorphous phase.
 5. The retroreflective sheet of claim4 wherein the beads comprise compositions selected from the groupconsisting of zirconia-silica and zirconia-alumina-silica.
 6. Aretroreflective sheet comprising:a resilient polymeric base sheet havinga front surface; a plurality of integral protrusions projecting from thefront surface, there being a plurality of such protrusions across thewidth of the sheet, each of the protrusions having a top surface and atleast one side surface connecting the top surface to the front surfaceof the base sheet; a first discontinuous layer of bead bond covering aselected set of side surfaces of the protrusions; a first plurality ofretroreflecting beads partially embedded in the layer of bead bond andpartially protruding from the layer of bead bond; a second layer of beadbond covering a second selected set of side surfaces of the protrusions;and a second plurality of particles partially embedded in the secondlayer of bead bond, the combination of said second bead bond and saidsecond plurality of particles being substantially different from thecombination of said first layer of bead bond and said first plurality ofretroreflective beads.
 7. The retroreflective sheet of claim 6 whereinthe embedded particles are retroreflective beads and the bead bond onthe first selected set of side surfaces is a different color from thesecond layer of bead bond covering the second selected set of sidesurfaces.
 8. The retroreflector sheet of claim 6 wherein the particlesare chosen from the group consisting of retroreflecting beads and skidprevention particles.
 9. The retroreflector sheet of claim 6 furthercomprising:a third layer of bead bond covering the top surfaces of theprotuberances; and a plurality of skid prevention particles partiallyembedded in the third layer of bead bond.
 10. A retroreflective sheetwhich comprises:a resilient polymeric base sheet having a front surface;a plurality of integral protrusions projecting from the front surface,there being a plurality of such protrusions across the width of thesheet, each of the protrusions having a top surface and at least oneside surface connecting the top surface to the front surface of the basesheet; a first discontinuous layer of bead bond covering a selected setof side surfaces of the protrusions; a first plurality ofretroreflecting beads partially embedded in the first layer of bead bondand partially protruding from the first layer of bead bond; a secondlayer of bead bond covering the top surfaces of the protrusions; and aplurality of skid prevention particles partially embedded in the secondlayer of bead bond.